Fix a potential issue spotted by Shasha. An IllegalArgumentException may end up in an infinite loop from Apache Flume. That would never happens right now since we check for null event even before going there, but it's better to keep the entire...

Summary: ...code safe in case we refactor the code and expose that issue by mistake

Test Plan: mvn clean install

Reviewers: shchen

Subscribers: nsafonova, shchen

Differential Revision: http://phabricator.tango.corp/D60